Connected: An Internet Encyclopedia
RFC 792 - Source Quench Message

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### Source Quench Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             unused                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Internet Header + 64 bits of Original Data Datagram      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       IP Fields:

       Destination Address

          The source network and address of the original datagram's data.

       ICMP Fields:

       Type

          4

       Code

          0

       Checksum

          The checksum is the 16-bit ones's complement of the one's
          complement sum of the ICMP message starting with the ICMP Type.
          For computing the checksum , the checksum field should be zero.
          This checksum may be replaced in the future.

       Internet Header + 64 bits of Data Datagram

          The internet header plus the first 64 bits of the original
          datagram's data.  This data is used by the host to match the
          message to the appropriate process.  If a higher level protocol
          uses port numbers, they are assumed to be in the first 64 data
          bits of the original datagram's data.

       Description

          A gateway may discard internet datagrams if it does not have the
          buffer space needed to queue the datagrams for output to the next
          network on the route to the destination network.  If a gateway
          discards a datagram, it may send a source quench message to the
          internet source host of the datagram.  A destination host may also
          send a source quench message if datagrams arrive too fast to be
          processed.  The source quench message is a request to the host to
          cut back the rate at which it is sending traffic to the internet
          destination.  The gateway may send a source quench message for
          every message that it discards.  On receipt of a source quench
          message, the source host should cut back the rate at which it is
          sending traffic to the specified destination until it no longer
          receives source quench messages from the gateway.  The source host
          can then gradually increase the rate at which it sends traffic to
          the destination until it again receives source quench messages.

          The gateway or host may send the source quench message when it
          approaches its capacity limit rather than waiting until the
          capacity is exceeded.  This means that the data datagram which
          triggered the source quench message may be delivered.

          Code 0 may be received from a gateway or a host.

Next: Redirect Message

Connected: An Internet Encyclopedia
RFC 792 - Source Quench Message